Washing machines are commonly used to wash laundry or other objects by using friction between water and the laundry in combination with emulsification of detergent. A modern automatic washing machine is capable of automatically performing washing, rinsing and dewatering in an integrated process once it is started.
Depending on the type of the washing tub used for containing laundry, washing machines may be classified into types including a top-loading type (in which a washing tub is upright) and a drum type (in which a washing tub is positioned horizontal). As compared with the top-loading type, a drum type washing machine generally has reduced overall height and increased washing capacity and causes less fabric twist. Thus, the drum type washing machine has become more popular.
A washing machine typically has a detergent supply device for dispensing detergent during a washing cycle. The amount of detergent to be dispensed may depend on the laundry load. When supplying washing water into a tub, the detergent supply device supplies detergent to the tub where water is also supplied, the detergent being solid-phase (e.g., powder) or liquid-phase, etc. The detergent dispensing systems in washing machines may have quite different configurations depending what kind of detergent can be used.
In a washing machine using powder detergent, the detergent supply device can only be installed in the upper portion of the tub as it would not be easy to move powder detergent upward if it was added at a lower position. In contrast, a washing machine using liquid detergent has no such limitation because liquid detergent loaded at a lower position can be brought upward by using a pump or the like. For example, in a washing machine using liquid detergent, a detergent supply device may be installed under a tub.
Liquid detergent is easily diluted in washing water and outperforms powder detergent in emulsification. Thus, liquid detergent is widely used. A detergent supply device in a washing machine may include a reservoir tank capable of storing liquid detergent. Since it is difficult for a user to directly see the level of liquid detergent in the reservoir tank, a level sensor can be used in the detergent supply device to ensure adequate liquid detergent for a washing operation.
According to a conventional washing machine, if liquid detergent is above a predetermined level in the reservoir tank, a level sensor is electrically conducted by the liquid detergent. If the liquid detergent is below the predetermined level, electric conductivity of the level sensor is cut off. Thus, a warning of shortage of liquid detergent may be sent to a user based on the conductivity of an electrode.
However, contact between the level sensor and liquid detergent likely causes corrosion and rust on the electrode. A level sensor in the conventional detergent supply device tends to suffer reduced sensing ability and accuracy due to scale accumulation on the electrode.